The fundamental reason is to resist Britain's endless oppression of the colonies and to be independent. (To put it bluntly, it is for your own benefit)
The American War of Independence seems to be a war between the British and the British, but the British skirmishers on the American mainland have to deal with the so-called world's strongest regular army sent by the British government. It seems that their courage is really commendable and admirable, because obviously not only many people will die, but also there is a great possibility of failure. Fortunately, however, Britain's old enemy and bitter enemy, the French, also joined in. France almost used a lot of troops to fight against Britain all over the world, to contain British troops and to aid American weapons. So France is America's benefactor.
The seven-year war between Britain and France played a great role in the American War of Independence. However, it is said that at first, the French didn't think much of Americans. Later, Washington won two battles, and France immediately changed its mind and felt that it would help the United States and declare war on Britain. France used its own army and fleet to fight Britain in the major colonies and oceans around the world, of course, because France was very worried at this time that if Britain defeated the United States, which wanted independence, it could dominate North America. In this way, Britain will truly become the dominant force. Moreover, France's previous defeat and humiliation in Britain made the French indignant. In any case, they don't want to see the British really succeed before they decide to go to war with Britain.
Before the Seven Years' War in Britain, both Britain and France had colonies in Canada, but later the British attacked the French-occupied areas and almost unified Canada. Not only Canada, many colonies in Africa and Asia, but also colonial islands were taken away by the British, so when I remember, France was not popular. That's why it took seven years to play so hard.
France and Britain fought for 130 years because of the royal family. In the end, Britain lost, not only suffered heavy losses, but also lost the original Normandy and other territories, and gained nothing. It is said that the only advantage of this war is to make the British people fully realize that the route of conquering Europe is completely unworkable. Only by managing the island well, there is only one way to go to the ocean wholeheartedly, which is very beneficial for Britain to dominate the global ocean in the future. The French benefited from this hundred-year war by being completely unified into one country-France, which created good conditions for France to become the largest power in the European continent in the future.
Later, Britain developed well at sea and had a large number of colonies and troops overseas, so Britain had little influence on the European continent and little intervention in Europe. I don't think the British want to interfere in the affairs of continental Europe. I think it has something to do with the lessons it learned from the Hundred Years' War. It only wants balance and peace in the European continent, because almost all its thoughts are focused on managing overseas colonies, because the oil and water in this colony is too great. Later, with the rise of Germany, of course, in order to maintain the balance of the European continent and enable it to operate overseas colonies safely, the British naturally stood on the opposite side of the strongest road in Europe forever, so that they stood on the same front as the French. At this time, the Russians were bent on expanding their territory in Europe. They met the powerful Prussia in Poland, and then Germany. This situation is a little close to that before the first world war.